<?php
class Q_Filter_Digits extends Q_Filter_Abstract {

    /**
     * The parameters that were passed to this filter
     */
    protected $_params = array(
        'field' => 0, // default is zero because this allows $filter->filter('some text')
    );
    public function doFilter($value) {
    
        return preg_replace('/[^0-9]/', '', $value);
    
    }

}